How to show an object in viewport as it is in another keyframe, without going to that other keyframe?
Blender Does not support anything as direct as Onion-skinning like in 2D Animation (without addons at least)
Blender DOES on the other hand, support Motion paths, Which will show you essentially the same information in a more basic way. To Use it, select the object, and find motion paths in the object info tab

Click Calculate, And it should give you a result like this:

Hopefully this helps!